This application is based on and claims priority under 35 U.S.C. xc2xa7 119 with respect to Japanese Patent Application No. 2002-068443 filed on Mar. 13, 2002, the entire content of which is incorporated herein by reference.
The present invention relates to a control method for a lockup clutch for controlling a slip rotation speed of a pump impeller and a turbine runner of a hydraulic power transmission mechanism. More particularly, the present invention pertains to a control method for a lockup clutch for determining a time for performing a pre-charge.
With a known lockup clutch provided on a torque converter, lockup control or slip control between a pump impeller and a turbine runner is performed by controlling pressure from a piston by controlling supplied hydraulic pressure. A time lag is generated from the hydraulic pressure to actually operating a friction plate of the lockup clutch to start engaging the pump impeller and the turbine runner due to a loss stroke of the piston. Provided that the time lag is increased, a response to reach the lockup control or the slip control is deteriorated. In order to restrain the influence of the loss stroke, the piston is operated swiftly by increasing the supply speed of the hydraulic pressure during pre-charging. However, because the greatness of the loss stroke of the piston is varied by abrasion of the lockup clutch or by change of properties with time, a predetermined pre-charge time becomes not applicable, which may cause a shock at the engagement of the lockup clutch.
Another known lockup clutch for obviating the foregoing drawback is disclosed in Japanese Patent Laid-Open Publication No. H5(1993)-296337. According to the known lockup clutch disclosed in Japanese Patent Laid-Open Publication No. H5(1993)-296337, a torque sensor for detecting an output shaft torque of an automatic transmission is provided in a system in which a torque converter and a lockup clutch are arranged in parallel in order to maximally shorten a loss stroke time of the piston within a range not generating the engagement shock of the lockup clutch. Waveform balance of an initial pressure waveform and of a final pressure wave balance during the pre-charge is amended so that the operation result of the piston by the pre-charge corresponding to a predetermined request using an output signal of the torque sensor.
Notwithstanding, because the waveform balance during the pre-charge is amended using the output shaft toque of the automatic transmission with the known lockup clutch disclosed in Japanese Patent Laid-Open Publication No. H5(1993)-296337, the lockup clutch is once suddenly engaged at the amendment of the waveform balance. Thus, the engagement shock of the lockup clutch may be caused during the amendment of the waveform balance.
A need thus exists for a lockup clutch and a control method of the lockup clutch which restrains the deterioration of the response performance due to the loss stroke of the lockup clutch and the engagement shock of the lockup clutch.
In light of the foregoing, the present invention provides a control method of a lockup clutch which includes eliminating a loss stroke of a piston by a pre-charge of a supplied hydraulic pressure, increasing a force applied to the piston by further supply of the hydraulic pressure after the pre-charge, controlling a slip rotation speed between a pump impeller and a turbine runner in a torque converter connected to an automatic transmission, and determining a time for performing the pre-charge when a shift stage of the automatic transmission is under a neutral state and a vehicle is substantially stopped.